Distributed sensing of earthquakes and ocean-solid Earth interactions on seafloor telecom cables

Abstract

Two thirds of the surface of our planet are covered by water and are still poorly instrumented, which has prevented the earth science community from addressing numerous key scientific questions. The potential to leverage the existing fiber optic seafloor telecom cables that criss-cross the oceans, by using them as dense arrays of seismo-acoustic sensors, remains to be evaluated. Here, we report Distributed Acoustic Sensing measurements on a 41.5 km-long telecom cable that is deployed offshore Toulon, France. Our observations demonstrate the capability to monitor with unprecedented details the ocean-solid earth interactions from the coast to the abyssal plain, in addition to regional seismicity (e.g., a magnitude 1.9 micro-earthquake located 100 km away) with signal characteristics comparable to those of a coastal seismic station.

Fig. 1. Map and perspective view of the seafloor MEUST-NUMerEnv cable.

The optic fiber cable offshore Toulon, France, shown in a map view and b 3D view. The 41.5 km long optical fiber crosses several oceanic domains: the shallow continental shelf, the steep continental slope and the deep oceanic plain. The yellow star on the map indicates the location of a magnitude 1.9 earthquake. The red triangle is the permanent seismic station POSA.

Fig. 2. Seismic seafloor signal caused by oceanic surface gravity waves.

a Depth profile and b 550 s long record of strain-rate along the first 10 km of the cable. Each trace is normalized by its maximum amplitude. The data show periodic oscillations mainly propagating towards the shore. c Mean strain-rate over the same distance as a function of depth and theoretical prediction for intermediate depth regime and a wavelength of 100 m. d Frequency-wavenumber f–k decomposition of the strain-rate signal, for seaward (k < 0) and landward (k > 0) components, and linear gravity wave dispersion curves for two different incidence angles assuming a water depth of 100 m (dashed curves).

Fig. 3. Observations of oceanic secondary microseismic noise.

a Depth profile of the OF cable. b 60 s record of strain-rate between km 32 and 40. c f–k decomposition of the signal. The dotted lines are the modeled frequencies of peak amplitude (minimum group velocity) of the fundamental mode of Scholte waves assuming a regional velocity model (Supplementary Fig. 2) and varying the P-wave velocity in the top sedimentary layer (1500 m/s in red and 2000 m/s in blue). d Depth profile of the OF cable along its full length. e Spectrum of the noise along the cable. The pressure directly produced by the swell vanishes at a depth that depends on frequency as predicted by linear theory (blue dotted curve shows predicted depth of 95% amplitude loss). Beyond 8 km from the coast, at depths larger than 200 m, the OF cable senses the second-order pressure fluctuations caused by the sea surface waves. In deep water, the frequencies of maximum energy are consistent with those predicted from the water column resonance effect that amplifies the Scholte waves (red dashed curve).

Fig. 4. Comparison of the M1.9 earthquake recorded at land station POSA and along the fiber optic cable.

a Bathymetry map of the OF cable and location of the broadband land station POSA (red triangle). DAS data are plotted either as strain-rate absolute amplitude filtered between 1 and 15 Hz measured every 10 m b, or as strain-rate averaged along 320 m sections of the cable c. The shaded portion in c indicates the section of cable with high amplifications and where the OF cable crosses a complex bathymetry. On top are plotted for comparison the three-component records of the on-land seismic station POSA. The two plots on the right side show the cable depth and epicentral distance at different distances along the cable.